The spine includes a series of joints routinely called motion segment units, which are the smallest component of the spine that exhibits kinematic behavior characteristic of the entire spine. The motion segment unit is capable of flexion, extension, lateral bending, and translation. The components of each motion segment unit include two adjacent vertebrae and their apophyseal joints, the intervertebral disc, and the connecting ligamentous tissue. Each component of the motion segment unit contributes to the mechanical stability of the joint.
Components of a motion segment that move out of position or become damaged can lead to serious pain and may lead to further injury to other components of the spine. Depending upon the severity of the structural changes that occur, treatment may include fusion, discectomy, or laminectomy.
Underlying causes of structural changes in the motion segment unit leading to instability include trauma, degeneration, aging, disease, surgery, and the like. Thus, rigid stabilization of one or more motion segment units may be an important element of a surgical procedure in certain cases (i.e., injuries, deformities, tumors, etc.), whereas it is a complementary element in others (i.e., fusion performed due to degeneration). The purpose of rigid stabilization is the immobilization of a motion segment unit.
FIG. 6 shows the anatomy of a typical vertebra 100. The vertebra 100 includes a vertebral body 101. An intervertebral disc 130 is supported by the vertebral body 101. The intervertebral disc 130 includes a disc annulus 131 surrounding nucleus pulposus 132. The vertebra includes number of processes: the transverse process 102, superior articular process 103, inferior articular process 104, and the spinous process 105. The transverse process 102 includes an anterior tubercle 106 and a posterior tubercle 107. The transverse process 102 has a foramen transversium 108 formed therein. The vertebra 100 has a foramen 109 formed therein. The spinal cord 120 runs vertically in the foramen 109. The meninges 121 surrounds the spinal cord 120. Within the spin cord 120, gray matter 122 is surrounded by white matter 123. Nerve roots 124 exit the spinal cord 120 and descend laterally. The nerve roots 124 partially cover a posterior approach to the vertebral body 101. Each nerve root includes a dorsal root 125 and ventral root 126.
As mentioned above, current surgical techniques typically involve fusing one or more unstable motion segment units and possibly, the removal of ligaments, bone, disc, or combinations thereof included in the unstable motion segment unit or units prior to fusing. There are several disadvantages to fusion, however. For example, the fusing process results in a permanent or rigid internal fixation of all or part of the intervertebral joints and usually involves the implantation of metallic rods, plates, and the like for stabilization. In all cases, the systems are intended to rigidly immobilize the motion segment unit to promote fusion within that motion segment unit.
When inserting a prosthesis into an intervertebral space via a posterior approach, the nerve roots can be damaged. These nerve roots from the adjacent vertebra are spaced apart at a distance narrower than the width of the intervertebral disc. The nerve roots block the posterior of the intervertebral space. To insert a prosthesis that is as wide as the intervertebral disc, the nerve roots are retracted.
Vertically expandable intervertebral prostheses are known. Most of these devices employ mechanical means to expand their height. The device is placed into an intervertebral space and then expanded vertically to support the adjacent vertebrae.